Digestive System Overview

Digestive System

Functions

• Ingestion & Chewing: Involves the initial intake of food and the mechanical breakdown of food through chewing.

• Secretion of Water, Acids, Enzymes, & Buffers: Various fluids and substances are secreted into the digestive tract to aid in food processing.
    - Water: Helps dissolve substances to assist in digestion.
    - Acids: Aid in digestion and provide a suitable environment for enzymatic actions.
    - Enzymes: Catalyze the breakdown of food into absorbable nutrients.
    - Buffers: Maintain optimal pH levels in various parts of the digestive system.

• Digestion into Small Organic Molecules: The mechanical and chemical breakdown of food into smaller components that can be absorbed.

• Absorption of Small Molecules into the Body: Nutrient absorption primarily occurs in the small intestine.

• Defecation of Waste and Undigested Material: Elimination of residual waste that the body cannot digest.

• Protection of the Body: The body treats the lumen of the digestive system similarly to the external environment by employing protective mechanisms.
    - The continuity of epithelial cells prevents unwanted substances from entering the internal environment.

Layers of the Digestive System

• MUCOSA: Innermost layer consisting of:
    - EPITHELIUM: Lining the lumen of the digestive tract.
    - LAMINA PROPRIA: Connective tissue layer supporting the epithelium.
    - MUSCULARIS MUCOSAE: Thin muscle layer aiding in local movement.

• SUBMUCOSA: Connective tissue layer that supports the mucosa and contains blood vessels and nerves.

• MUSCULARIS EXTERNA: Responsible for the peristaltic movement, consisting of:
    - Circular Muscle Layer: Inner layer providing constriction.
    - Longitudinal Muscle Layer: Outer layer aiding in shortening of the digestive tract.

• SEROSA: Outermost layer consisting of connective tissue and an outer epithelium.

• Mucosa Associated Lymphoid Tissue (MALTs): Lymphoid nodules located in the mucosa providing immune protection.

Oral Cavity

Functions of the Oral Cavity

• Tasting: Analyzing potential foods through taste receptors.

• Chewing: Utilizing teeth, tongue, and palate to form a bolus—a soft, moist ball of food.

• Digestion of Sugars: Initiated by the enzyme SALIVARY AMYLASE, starting carbohydrate digestion.

Anatomy of the Oral Cavity

• Surrounded by:
    - Hard & Soft Palate
    - Tongue
    - Cheeks

• Gums (GINGIVA): Surround teeth, providing a protective structure.

• Continuous Lips (LABIA): Connect cheeks to the exterior.

Salivary Glands

• There are three pairs:
    - PAROTID Glands
    - SUBLINGUAL Glands
    - SUBMANDIBULAR Glands

• Saliva Functions:
    - Lubricates & cleanses the oral cavity.
    - Dissolves hydrophilic molecules for taste.
    - Suppresses bacterial growth.
    - Contains salivary amylase for initial carbohydrate digestion.

Pathology of the Mouth & Teeth

MUMPS

• Viral infection affecting the salivary glands.

• Following infection/immunization, permanent immunity develops.

• Can lead to complications, such as:
    - Sterility in adult males if testes are affected.
    - Possible infection of the pancreas, leading to diabetes.

• Vaccination efforts have reduced mumps incidence.

Lactose Intolerance

• Caused by insufficient lactase enzyme production as individuals age.

• After consuming lactose, undigested sugars ferment, causing gastric distress due to bacterial metabolism in the gut.

Teeth Structure

• Teeth Functions: Masticate food into smaller pieces to mix with saliva and form a bolus.

• TOOTH STRUCTURE:
    - ENAMEL: Hard outer surface covering the crown.
    - DENTIN: Softer inner material forming the bulk of the tooth shape.
    - ROOTS: Anchor the tooth in the jaw bone.
    - PULP: Connective tissue housing nerves and blood vessels in the tooth's center.
    - CEMENTUM: Covers the tooth roots; anchors the tooth in the socket via periodontal ligament.

Dental Pathologies

• Dental Plaque: Bacterial accumulation that can lead to tooth decay (caries).
    - Acids produced by bacteria dissolve minerals in teeth if not removed by regular brushing.

• Importance of Oral Hygiene:
    - Brushing is essential for plaque removal.
    - Flossing helps remove food particles and bacteria between teeth.
    - Neglect can lead to gingivitis, characterized by gum inflammation, bleeding, and bad breath.

• Periodontal Disease: Advanced stages of untreated gingivitis, leading to deepening of the gingival sulcus and potential tooth loss.

Functions of the Tongue

• Pushing Food: Assists in maneuvering food between teeth for chewing.

• Swallowing Assistance: Aids in the swallowing process.

• Taste: Taste receptors located in taste buds on tongue papillae detect flavors.

Pharynx

• Serves as a passageway for food, drink, and air. Muscles propel food toward the esophagus during swallowing.

• Divided into three regions:
    - Nasopharynx
    - Oropharynx
    - Laryngopharynx

Esophagus

• A muscular tube approximately 25 cm (10 inches) long located behind the trachea.

• Function: Only transport; does NOT facilitate digestion or absorption.

• Lined with stratified squamous epithelium for protective functions, unlike other digestion areas.

Swallowing Phases

1. Oral Phase: Food forms into a bolus and moved by the tongue to the back of the mouth.

2. Pharyngeal Phase: Tongue pushes down the epiglottis, closing off the trachea; food slides into the esophagus.

3. Esophageal Phase: Peristalsis moves the bolus toward the stomach.

Peristalsis

• Definition: Muscle contractions that propel food forward through the digestive tract.

• Involves:
    - Circular Muscles: Contract behind bolus, relax in front.
    - Longitudinal Muscles: Contract to shorten the digestive tract aiding food movement.

Stomach

• Function: Receives food, processes it, and prepares it for the small intestine.

• Controlled by CARDIAC SPHINCTER (top) and PYLORIC SPHINCTER (bottom).

• Secretions:
    - Mucus: Protects stomach lining.
    - Hydrochloric Acid (HCl): Creates acidic environment for pepsin.
    - Intrinsic Factor: Required for vitamin B12 absorption.
    - Pepsinogen: Initiates protein digestion.
    - Gastric Lipase: Starts lipid digestion.
    - Gastrin: Hormone that regulates stomach processes.

• Absorption: Nutrients are not absorbed in the stomach.

Stomach Cells

• Goblet Cells: Produce protective mucus.

• Chief Cells: Secrete pepsinogen and gastric lipase.

• Parietal Cells: Produce hydrochloric acid and intrinsic factor.

• Endocrine G Cells: Produce gastrin.

• Renewal of epithelial cells occurs in the neck region of gastric glands.

Stomach Regions

• Cardias

• Fundus

• Body

• Pylorus: Opens to the duodenum via the pyloric valve.
    - Curvatures: Greater and lesser omenta attached at the greater and lesser curvatures respectively.

Accessory Organs of the Small Intestine

• Liver: Produces bile for lipid emulsification.

• Gallbladder: Stores and concentrates bile.

• Pancreas: Produces digestive enzymes and bicarbonate for neutralizing stomach acid.

Liver Anatomy

• Largest gland in the body weighing approximately 3 lbs.

• Comprised of four lobes:
    - Right Lobe
    - Left Lobe
    - Caudate Lobe
    - Quadrate Lobe

• Falciform Ligament: Supports the liver from the diaphragm and anterior abdominal wall.

• Round Ligament: Remnant of the fetal umbilical vein, running along the free edge of the falciform ligament.

Liver Structure

• Porta Hepatis: Allows passage of:
    - Hepatic Portal Vein: Carries nutrient-rich blood from the intestines.
    - Hepatic Artery Proper: Supplies oxygen-rich blood.
    - Common Hepatic Duct: Conducts bile produced by the liver.

Liver Lobule

• Basic structural and functional units of the liver, shaped hexagonally.

• Comprising:
    - Hepatocyte Plates: Filter and process blood.
    - Central Vein: Central axis of the lobule.
    - Portal Triad (each corner): Contains:
      - Branch of hepatic artery.
      - Branch of hepatic portal vein.
      - Bile duct that collects bile from hepatocytes.

Gallbladder

• Muscular sac located on the ventral surface of the liver.

• Function: Stores and concentrates bile by absorbing water and ions.

Biliary Tree

• Composition:
    - Common Hepatic Duct: Exits liver.
    - Cystic Duct: Connects gallbladder with the bile duct.
    - Bile Duct: Formed by the merger of common hepatic and cystic ducts.

Pancreas

• Mostly situated retroperitoneally, behind the stomach's greater curvature.

• Head: Surrounded by the duodenum.

• Functions:
    - Exocrine: Produces pancreatic juice for digestion.
    - Endocrine: Secretes hormones like insulin and glucagon from the pancreatic islets.

Small Intestine

• Major organ for digestion and absorption, measuring 2-4 m (7-13 ft) long.

• Subdivisions:
    - Duodenum: Primarily retroperitoneal.
    - Jejunum: Attaches posteriorly via mesentery.
    - Ileum: Connects to the large intestine at ileocecal valve.

Small Intestine Structural Modifications

• Increase surface area for enhanced nutrient absorption:
    - Circular Folds: Increase surface area.
    - Villi: Projections that increase absorption area.
    - Microvilli: Tiny projections on villi further increase absorption surface area.

Large Intestine

Unique Features

• Teniae Coli: Longitudinal bands of smooth muscle in the muscularis helping to form haustra.

• Haustra: Pocket-like sacs formed by the contraction of Teniae coli.

• Epiploic Appendages: Fat-filled pouches of visceral peritoneum.

Subdivisions of the Large Intestine

• Cecum: Initial part; contains the appendix.

• Colons:
    - Ascending Colon: Travels up the right side of the abdomen.
    - Transverse Colon: Travels across the abdomen.
    - Descending Colon: Moves down the left side.

  - Sigmoid Colon: S-shaped portion before rectum.

Rectum: Contains three rectal valves preventing passage of gas with feces.

• Anal Canal: Final segment with two sphincters:
    - Internal anal sphincter: Smooth muscle, involuntary control.
    - External anal sphincter: Skeletal muscle, voluntary control.

Processing of Nutrients

Digestion of Carbohydrates

• Begins in the mouth with salivary amylase.

• Only monosaccharides are absorbable by the body.

• Steps in Small Intestine:
    - Maintained by pancreatic amylase, breaking starch into oligosaccharides and disaccharides.
    - Brush border enzymes further degrade these into monosaccharides.

Digestion of Proteins

• Proteins are divided into:
    - Large polypeptides→small peptides→amino acid monomers.

• Digestive process starts in the stomach with conversion of pepsinogen to pepsin at pH 1.5-2.5.

• Pepsin becomes inactive under higher pH conditions of the duodenum.

Digestion of Lipids

• Starts with emulsification by bile salts to reduce fat globules into smaller droplets.

• Digestive process follows:
    - Pancreatic lipases convert fats into monoglycerides and free fatty acids.
    - Micelle Formation: Digestion products become coated with bile salts and lecithin.
    - Diffusion: Lipid products diffuse across epithelial membranes.
    - Chylomicron Formation: Resynthesized into triglycerides, packed with lecithin and lipoproteins to form chylomicrons.
    - Transport: Chylomicrons enter embryological lacteals for lymphatic transport.

Digestion of Nucleic Acids

• Pancreatic nucleases hydrolyze dietary nucleic acids (DNA/RNA) into nucleotide monomers.

• Brush Border Enzymes: Nucleosidases and phosphatases break nucleotides down into free nitrogenous bases, pentose sugars, and phosphate ions.

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